Method of and device for recovering energy when cooling compressed gases in heat exchangers



Mai'ch 3, 1959 F. A. METHOD OF AND DEVICE FO HORN 2, R RECOVERING ENERGYWHEN COOLING COMPRESSED GASES IN HEAT EXCHANGERS Filed July 10, 1956 1:)no so i heal/kg United States Patent METHOD OF AND DEVICE FOR RECOVERING ENERGY WHEN COOLIN G COMPRESSED GASES IN HEAT EXCHANGERS Fritz A.Horn, Essen, Germany, assignor to Ruhrgas Aktiengesellschaft, Essen,Germany Application July 10, 1956, Serial No. 597,018 Claims priority,application Germany July '20, 1955 7 Claims. (Cl. 62-87) to escape intothe atmosphere without furnishing any useful work.

It is also known to use the compression heat in heat exchangers for thepurpose of preheating combustion air, for increasing the naturalventilation in mines, and for heating, drying and other thermalpurposes. When in this way exploiting the compression heat for obtainingsensible.

heat, frequently the drawback is encountered that the consumption of theobtained heat does not coincide with the obtainment of such heat so thatthe compression heat can be obtained and exploited only in part and isnot present when the compressor is stopped.

- It is, therefore, an object of the present invention to provide amethod and device which will overcome the above mentioned drawbacks.

It is another object of this invention to provide a method of and devicefor recovering energy which is freed during the cooling of gasescompressed in one or more stages, which will make it possible to carryout a circulation of the heat transfer fluid at high pressure. object ofthis invention to provide a method and device as set forth in thepreceding paragraph which will make it possible to effect an effectiverecovery of energy by means of relatively small devices and expandingmachines and which will work at a high degree of efliciency.

These and other objects of the invention will appear" more clearly fromthe following specificationin connec tion with the accompanying drawingsin which:

Fig. 1 is a diagrammatic illustration of an arrangement for carrying outthe method according to the invention.

Fig. 2 illustrates the p-i graph.

According to the present invention, the cooling of gases compressed inone or more stages is carried out in heat exchangers which are arrangedsubsequent to the compression stages in such a way that a vapor isemployed as heat transfer fluid in the heat exchangers which vapor has alow critical temperature lower than the temperature of the compressedgas to be cooled, but higher than atmospheric temperature. Furthermore,the heat transfer fluid is passed into the heat exchangers at a pressureabove the critical pressure. Furthermore, in the heat exchangers theheat transfer fluid is superheated at least It is still another'2,875,589 Fatented Mar. 3, 1959 to such an extent that at least half theexpansion of the superheated heat transfer fluid which is fed to anexpansion machine occurs within the range of the superheating, therebycausing the heat transfer fluid to perform work. Thereupon the heattransfer fluid is conveyed to a condenser with subsequent pump in whichthe condensed heat transfer fluid is placed under tension preferably farbeyond the critical pressure. Finally theheat transfer fluid is conveyedby the pump again into the heat ex changers.

The method according to the invention thus makes it possible to carryout the circulation of the heat transfer fluid at high pressure and dueto the small vapor volume of the heat transfer fluid inherent theretoitis possible, to

carry out the method with relatively small devices and The methodaccording to the invention may be carried,

out in different ways as will be seen from Fig. 1. The heavy line inFig. 1 indicates the course of the compressed gas, while the dot-dashline indicates the path of the heat transfer fluid. The arrangementcomprises gas compressors 1 and 2. arranged in series followed by heatexchangers 1a and 2a. The reference numeral 3 designates a deep-coolerdesigned as heat exchanger, whereas the reference numeral 4 designates apump driven by an electric motor. The arrangement shown in Fig. 1furthermore comprises an expansion machine 5, a cooling device 6 cooledby cooling water for the heat transfer fluid, a choke nozzle 7, anelectric prime mover 8 for the compressors 1 and 2, a compressor 9 forthe heat transfer fluid, and a precooler 10 designed as heat exchanger.

The prime mover 8, the expansion machine 5, the compressors 1 and 2, andthe compressor 9 for the heat transfer fluid are arranged on a shaftcommon to all of them. In this way mechanical transmission losses areavoided.

The operation of the arrangement diagrammatically shown in Fig. 1 is asfollows: a p

The gas to be compressed, for instance long distance gas, passes throughthe compressor 1- -first compression stageis then cooled in the heatexchanger 1a, is furthermore compressed in the compressor 2-sec0ndcompression stage-and is again cooled in the a heat exchanger 2a. Thegas may then be further cooled in the deep-cooler 3 in a mannerdescribed further below while the ingredients condensing during thedeep-cooling are removed, The thus obtained gas may then be heated againin the precooler 10 where the gas cools the gas being fed to thedeep-cooler.

The heat transfer fluid, for instance CF CI passes at a pressure abovethe critical pressure from the pump 4 into heat exchangers 1a and 2awhere it is evaporated and highly superheated. Thereupon, it passesthrough the expansion machine 5 in which it expands and performs workwhile the expansion is effected preferably only within the range of thesuperheating. The heat transfer fluid is then condensed in the watercooled cool: ing device 6 and from there returns to the pump 4. A

The heat transfer fluid then passes through thecompressor 9 for the heattransferfluid in which it is cornw designed as heat exchanger pressedand pumped into the feeding line for the heat transfer. nus.pertainingto. the. cooling devicev 6.

If it is not desired to deep-cool the gas, the choke nozzle 7, thedeep-cooler 3, the compressor 9 for the heat transfer fluid and theconduit for the cooling means interconnecting said parts, as well as theprecooler it may be omitted.

The method according to the invention yields an exr mc y hi h. xpl i aon f h c mpr s on a Which fact isprirnarily due to the highsuperheatingofthe heat transfer fluid in the heat exchangers 1n and 2a and is alsodue to the fact that subsequently the heat energy of the. heat transferfluid is obtained as mechanical energy in the expansion machine withinthe range of expansion preferablyfonly within the range of superheatingI I The. method. according to the present invention makes it possible atrelatively low investment costs for the arrangement, which costs arepartially compensated for by the reduced size of, the prime mover 8 forthe compressors 1 and 2, to obtain a permanent reduction in the.consumption of energy for the gas compression which reductionhas-heretofore appeared to be impossible.

Fig. 2 of the drawing illustrates the p-i graph for the heat transferfluid CFgClg- The lines 12-3-41 illustrate an example of the circuit tobe passed through by said heattransfer fluid. At point 1 the heattransfer fluid is in liquid condition at approximately 30 C. and at apressure of 7 kilograms per square centimeter. By means-of the pump 4 ofFig. 1, the heat transfer fluid .is brought up to a pressure of 55kilograms per square centimeter (point 2). When the heat transfer fluid;passes through the heat exchangersla and 2a, its

temperature increases steadily to about 140 C. (point 3) and in this wayis brought into a gaseous; superheated condition. The steady increase intemperature at which the evaporation above the critical point iseffected in contrast to the evaporation within the evaporating range, isof particular importance for the heat exchange in countercurrent withthe gas to be cooled in the heat exchanger. The said increase intemperature makes possible an even temperature drop from the gas to theheat transfer fluid over the entire path through the heat exchanger,which fact materially simplifies the construction of the heat exchanger,and brings about a reduction in size and in costs.

From point 3 to point 4 the adiabatic expansion ocours in the expansionmachine 5. From point 4 back to point 1 the condensation in the coolingdevice 6 occurs through the range of the wet vapor.

The lines 156-7-1 illustrate an example for the circuit passed throughby the branch current which may be branched oif ahead of the pump 4 andemployed for deep-cooling thegas While the heat content i is maintained,the branch current expands in the nozzle 7 to a pressure of 3 kilogramsper square centimeter and to a temperature of approximately 1 C. (point'while a partial evaporation occurs. The branch current is completelyevaporated (point 6) in the deep-cooler 3 by heat exchange with thecompressed gas whereupon the branch current by means of the compressor 9is again compressed to a pressure of 7 kilogramsper square centimeterand is heated up to approximately 30 C. (point- 7) and finally is againliquified in the cooling device 6 (point 1). In a similar manner, the,above, described circuits can be carried out with carbon dioxide,propane, isobutane, or ethane.

It is, of course, understood that the present invention is, by no means,limited to the particular arrangement shown in the drawing but alsocomprises any modification within the scope. of the appended claims.

' What'I claim is:

l. A method of recovering energy from the'heat which is freed in heatexchanger means during the cooling of through said choke nozzle,thereupon compressing said,

compressed gases, which includes the steps of :employing a heat transferfluid. which. has. a. critical empcratue.

lower than the temperature of said compressed gases but higher thanatmospheric temperature and conveying said heat transfer fluid inliquefied form to said heat exchanger means at a pressure higher thanthe critical pressure, superheating said heat transfer fluid in saidheat exchanger means to at least such an extent that at least half theexpansion of said superheated heat transfer fluid when subsequentlyexpanding occurs within the superheated range, expanding saidsuperheated heat transfer fluid to thereby cause the same to performwork, condensing said expanded heat transfer fluid, placing said heattransfer fluid under a pressure higher than the critical pressure, andagain conveying said heat transfer fluid to said heat exchanger means.

2. A method of recovering energy from the heat which is freed in heatexchanger means during the cooling of compressed gases, which includesthe steps of: introducing into said heat exchanger means at a pressurehigher than the critical pressure a liquefied heat transfer fluid havinga critical temperature lower than the temperature of said compressedgases but higher than atmospheric temperature, superheating said heattransfer fluid in said heat exchanger meansto such an extent that saidsuperheated heat transfer fluid will be able to exp nd within the rangeof superheating when. being expanded; to perform work, expandingv saidsuperheated heat transfer fluid within said range. of superheating inanexpansion machine to obtain useful work, condensing said expanded heattransfer fluid, placing the heat transfer fluid under a pressure higherthan the critical pressure thereof and thereupon re-introducing saidheat transfer fluid into said heat exchanger means.

3. A'method according to claim 2, which includes the steps of: prior toplacing said heat transfer fluid under a pressure higher than thecritical pressure thereof; branching oif a portion of said heat transferfluid, passing said branched off heat transfer fluid through a chokenozzle, deep cooling said compressed gases discharged from said heatexchanger means by said heat transfer fluid passed heat transfer fluid,and finally rejoining the thus compressed branched off heat transferfluid with the heat transfer fluid discharged from said expandingmachine.

4. An arrangement for recovering energy from the heat which is freed isheat exchanger means during the cooling of compressed gases, whichcomprises in combination: first conduit means for said compressed gas,said conduit means having interposed therein heat exchanger meansarranged for receiving said compressed gas, and cooler means connectedto said heat exchanger means and provided with a discharge; and circuitmeans.

for the circulation of heat transfer fluid which. includes condensingmeans. for receiving vaporous heat transfer fluid and condensing thesame, second conduit means passing from said condensing means throughsaid heat exchanger means to cool the compressed gas in the: latterwhereby said heat transfer fluid which is conveyed to said heatexchanger means at a pressure higher than its critical pressureissuperheated, an expansion machine having an inlet and an outlet, thirdconduit means leading from said second conduit means at said heatexchanger means to said inlet for causingthe superheated heattransprising a restrictor, a deep-cooler arranged in series with saidrestrictor, and compressor means for said heat transfer fluid, saidcompressor means for said heat transfer fluid being arranged in serieswith said restrictor and said deep-cooler and communicating with saidcondenser 5 means for delivering compressed heat transfer fluid thereto.

7. An arrangement according to claim 4, in which the heat transfer fluidof said first conduit means comprise a deep-cooler having an outlet, 9.pre-cooler interposed between said heat exchanger means and saiddeep-cooler, and discharge conduit means connected to said deep-cooleroutlet and passing through said pre-cooler'.

References Cited in the file of this patent UNITED STATES PATENTS1,871,244 Steuart Aug. 9, 1932

